var str = process.argv[2];
if(typeof(str) == "undefined" || str == "--help" || str == "-h"){
    console.log("请输入一个数学表达式");
    process.exit();
}else{
    var math = str.substring(0, 3);
    try{
        console.log(eval(math));
        process.exit();
    }catch(err){
        console.log("命令行参数不是合法的数学表达式");
        process.exit();
    }
}